This invention relates to an electronic sphygmomanometer and, in particular, to a sphygmomanometer wherein accurate measurement of a patient's systolic and diastolic pressures are obtained without requiring the use of a sound transducer for detecting Korotkoff sounds.
It has long been known that an approximation of a patient's systolic blood pressure and diastolic blood pressure can be obtained by detecting the so-called Korotkoff sounds. Essentially, this measurement technique utilizes an inflatable occluding cuff which usually is wrapped about a patient's limb so as to close, or completely occlude, an artery. Typically, the occluding cuff is wrapped about the arm in juxtaposition to the brachial artery. When the cuff is inflated to a pressure which exceeds the patient's systolic pressure, so as to close this artery, blood is no longer capable of flowing therethrough. As the cuff is slowly deflated, a point is reached whereat the patient's systolic pressure exceeds the cuff pressure. Consequently, the artery opens for a short period during the patient's cardiac cycle. Once the blood pressure during this cardiac cycle falls below the cuff pressure, the artery once again is closed.
The pressure in the cuff which is equal to the maximum blood pressure during a cardiac cycle is, of course, the systolic pressure. It is known that when the blood pressure exceeds the actual cuff pressure, resulting in the opening of the artery, turbulence in the blood stream is accompanied by a sound which is the so-called Korotkoff sound. These Korotkoff sounds occur each time that the artery is opened. Thus, as long as the cuff pressure exceeds the lowest, or diastolic, pressure in the cardiac cycle, the artery will be alternately opened and closed as the cardiac cycle pressure traverses the cuff pressure. When the cuff pressure falls below the lowest pressure point in the cardiac cycle, the artery will remain opened, and the Korotkoff sounds no longer will be produced. Consequently, by measuring the cuff pressure at the last Korotkoff sound, a close approximation is made of the patient's diastolic pressure.
It is common practice to deflate the cuff at a rate which is much slower than the cardiac cycle. For example, the cuff is deflated at a rate in the range of 2mm Hg per second to 4mm Hg per second; so that it is expected that a Korotkoff sound will be present for each millimeter of mercury during the cuff deflation until the diastolic pressure is reached.
To detect the Korotkoff sounds, a suitable listening device has been required. For manual measurements of blood pressure, a stethoscope is applied to the patient's arm downstream of the occluding cuff. Because of the relative insensitivity of a conventional stethoscope, and further in view of ambient noises which can cause distraction or confusion in the detection of an actual Korotkoff sound, it is necessary for a physician or an otherwise skilled technician to take blood pressure measurements. This, of course, is an inefficient and generally wasteful use of a physician's time and skill.
Accordingly, there have been previous proposals for sphygmomanometers which can be used to measure blood pressure without the assistance of a physician or a highly skilled technician. In these earlier proposals, the detection of the Korotkoff sounds are achieved automatically and the associated cuff pressure readings are derived in conjunction with the detected Korotkoff sounds by electronic apparatus. It is generally believed that, prior to the instant invention, all of the earlier proposals and systems proceeded upon the detection of the Korotkoff sounds and, therefore, required the use of a microphone. Unfortunately, the automatic detection of Korotkoff sounds is accompanied by various problems and disadvantages. For example, the characteristic Korotkoff sounds of one patient may be vastly different from those of another patient. In particular, the amplitudes of these Korotkoff sounds may differ by many orders of magnitude. As another example, in some patients, during cuff deflation, but while the cuff pressure is between the patient's systolic and diastolic pressures, the Korotkoff sounds will appear to cease but then subsequently reappear. Since the finally detected Korotkoff sound is assumed to correspond to the patient's diastolic pressure, this interruption in the Korotkoff sounds will lead to erroneous measurements. As another example, background noises, known as artifactual noise to distinguish these noises from the Korotkoff sounds, can closely approximate such Korotkoff sounds and thus will be falsely detected. Attempts to overcome these, and other, problems are described in the patent art. For example, in U.S. Pat. No. 3,405,707, it is proposed to automatically simulate the selective process for discriminating Korotkoff sounds which is used by a physician. Nevertheless, this proposal requires the use of a microphone for sensing the Korotkoff sounds. Another proposal, described in U.S. Pat. No. 3,771,515, also relies on the use of a microphone.
Although it has been known that separate pressure signals are produced generally in phase with the Korotkoff sounds during a cardiac cycle, as mentioned in U.S. Pat. No. 3,349,763, nevertheless, there has been no previous attempt to detect these pressure signals and to use them in measuring blood pressure. In fact, even though pressure transducers have been used to sense cuff pressure, as described, for example, in U.S. Pat. Nos. 3,450,131 and 3,508,537, there has been no attempt to use a pressure transducer for sensing these pressure signals and for using same to measure systolic and diastolic pressure. Rather, the classic technique which relies upon the detection of the Korotkoff sounds, as described in U.S. Pat. No. 3,371,661, has been maintained.
Unfortunately, all of these systems which require the detection of the Korotkoff sounds are accompanied by many of the foregoing problems. Attempts to avoid the disadvantages inherent in sensing the Korotkoff sounds have not been entirely successful. Although the use of digital techniques has improved the measuring sensitivities, the unreliability of sound detection has tended to substantially nullify these improvements in accuracy. Although still better results can be achieved by using highly sensitive and discriminating microphones, the consequential increases in manufacturing costs and maintenance of the sphygmomanometer do not economically justify the use of such microphones.